One of the advantages of frequency-modulated (FM) transmission systems is the "capture" effect, the ability of an FM discriminator, such as a phase-locked loop or a ratio-detector, to demodulate "coherently", to "lock-on" the spectrum of a stronger signal spectrum and to completely ignore a weaker signal having a spectrum partially overlapping that of the stronger one. However, this advantage turns into a decisive disadvantage whenever the weaker signal is the desired one. Combatting interference due to such spectrum overlap currently requires inefficient use of available spectrum space by providing "guard bands" beyond the bandwidth required for information transmission and by geographic dispersion and reduced power of adjacent frequency transmission channels. Notwithstanding such costly precautions, the interference by stronger signals continues to be a problem in fringe areas or in mobile receivers.
A number of solutions have been proposed to mitigate or eliminate the capture effect. Thus Wilmotte, in U.S. Pat. No. 2,386,528 proposes cancellation of the stronger signal by selectively detecting two different signals, and relying on amplitude discrimination. The combination of the weak and strong signals, on emerging from a superheterodyning circuit of a receiver, is amplified in an intermediate frequency amplifier and clipped in a limiter; the amplified and clipped signal is combined with the signal emerging from the superheterodyning circuit in a combining circuit buffered or isolated from the receiver without making use of frequency discrimination, and is further amplified in a chain separate from the receiver. Wilmotte, in U.S. Pat. No. 2,388,200 purposes a solution applicable primarily to a strong frequency modulated signal in the presence of a weaker amplitude-modulated signal, where the detected output of the frequency modulated signal, due to the employment of a limiter and regenerative feedback, is increased with respect to that of the amplitude-modulated signal.
Harmon, in U.S. Pat. No. 2,609,535 discloses a system of transmitting two frequency-modulated signals over a common channel, and detecting the signals separately without mutual interference; as the signal waveforms of the transmitted signal must observe special relationships, Harmon's system is not, though, applicable to the capture effect.
Perkins, in U.S. Pat. No. 3,020,403 proposes a solution to the capture effect by converting the received frequency-modulated signals to amplitude-modulated signals, and processing the resultant signals to favor the weaker of the two signals.
Moore, in U.S. Pat. No. 3,091,735 proposes signal processing of two overlapping signals after a limiter postcoupled to a common intermediate frequency stage and uses a further limiter in a channel postcoupled to the first limiter, in addition to a variable gain amplifier, to select the weaker over the stronger signals.
Castellini, in U.S. Pat. No. 3,092,776, discloses a method and an apparatus for the reduction of interference by relying on the fact that given two waves, one of which is weaker relative to the other, the weaker wave may be obtained in association with a third wave, relative to which it is the stronger.
Ludwig, in U.S. Pat. No. 3,205,443, discloses an interfering signal resolving system relying on a tracking attenuator.
Baghdady, in U.S. Pat. No. 3,287,645, discloses circuitry for dividing the intermediate frequency signal into two channels. The first channel contains a beat frequency detector, the output of which is connected to one input of a balanced modulator, the other input of which is the intermediate frequency signal. The second channel includes a square law device which feeds one input of a second balanced modulator, the second input of which is the intermediate frequency signal. The output of the second balanced modulator is then passed through a bandpass filter. The output of the filter and the output of the first balanced modulator are then applied to an adder circuit, the output of which contains the desired signal at a sufficient amplitude relative to the undesired one to enable it to be captured by the succeeding FM limiter and detector.
Baghdady, in U.S. Pat. No. 3,911,366 discloses a receiver interference suppression technique and apparatus, relying on a first limiter in a first channel, a second limiter in the first channel, a limiter in a second channel, on a linear transfer device in the second channel, and on an output combiner of the two channels to suppress disturbances caused by certain troublesome forms of interference.
Ogita, in U.S. Pat. No. 4,249,261, discloses a superheterodyne radio receiver with nearby-station interference detection, in which a cancelling signal, generated by a phase-locked loop following, or attempting to follow, the strongest R.F. signal, is fed to the radio frequency stage of the receiver. Ogita employs detection means which are not frequency-dependent but deliver a detection signal representing an indication of the occurrance, in the receiver, of an interference by an adjacent-channel signal, whenever there is detected the condition that the RF signal level exceeds a predetermined reference level, and that, at the same time, the IF signal is lower than another predetermined reference level. The transition from one set of conditions to another is not continuous, but the phase-locked loop is switched in and out of the receiver circuitry by a switch according to the abovedescribed criteria. Discontinuous use of a phase-locked loop does, of course, usually present problems of acquisition, there being a finite acquisition time, which results either in undesirable switching transients, or other undesirable interference with reception of the desired signal.
The presence of the detection signal is a condition precedent to the operation of the phase-locked loop; in other words the phase-locked loop cannot properly operate in Ogita's receiver without the generation of the detection signal, which in turn controls the On-Off operation of the phase-locked loop.
Carpenter, in U.S. Pat. No. 3,753,123 discloses a technique and apparatus for cancelling FM modulated radio waves. Like Ogita, he generates a cancelling signal from a phase-lock loop following or attempting to follow the interfering FM signal. His invention does not solve the problem of distinguishing between two signals using the same modulation method for the purpose of cancelling only the interfering one. It appears to be intended for a different application, presumably to solve problems such as may occur in space vehicles where interference from radio waves carrying different types of modulated must be sorted out.